Turbine components of turbomachines, for example vanes of gas turbines (reference may be made in this regard, for example, to EP 0 620 362 B1), are exposed, by virtue of their design and function, to considerable thermal and mechanical stresses, on account of which damage, inter alia undesirable material deformations, may occur during the intended use of the machine. This applies particularly to turbine vanes. Such deformations of elements, in particular of elements moving in relation to an adjacent element, which arise as a result of thermal and mechanical load, may lead to adverse frictional and/or sealing losses. This may be detrimental to the efficiency of the machine and, for example, make it necessary to an increased extent, because of the stripping of material, to carry out an inspection of the turbo machine, focusing on specific wearing parts.
In simplified terms, a (gas) turbine has along an axis a compression portion with a compressor for a working fluid (for example, for gas, such as air), and, following downstream, at least one combustion chamber in which pressurized hot fluid, for example a hot gas, is generated by the admixing and ignition of fuel, and then, following the combustion chamber, a turbine portion, by means of which mechanical performance is generated, along with a partial expansion of the abovementioned hot fluid.
A plurality of rows or rings of (rotating) moving blades and (stationary) guide vanes are provided in the turbine in the direction of flow, for example in an alternating arrangement. For routing the flow of the working fluid and to achieve high efficiency in the transmission of pulses from the working fluid to the moving blades, the turbine unit usually comprises a number of stationary guide vanes. These are fastened in the form of a ring to an inner casing or to the stator of the turbine. Moving blades serve for driving the turbine shaft by the transmission of pulses from the working fluid flowing through the turbine.
These vanes are therefore either fastened, for example, to a rotor at a base plate, called a vane root, and extend radially outward with a vane body or airfoil or, for example fastened with the vane root to a stator, project radially outward with the vane body with respect to the axis.
The vane root of the guide vane can therefore be arranged on the inner casing of the turbine as a wall element in order to fix the respective guide vane and can form the outer boundary of a flow duct for the working fluid flowing through the turbine. At a distal end of the vane body, lying opposite the vane root, cover plates or shrouds can be connected for sealing purposes, as is known from the prior art. The flow duct can be delimited inwardly, and therefore clearly defined, by a ring shaped seat like shroud formed by cover plates lined up with one another. Moreover, a medium cooler than the abovementioned working fluid can flow around the flow duct, for example in order to limit a temperature of the corresponding turbine components.
In order to achieve as high a turbine inlet temperature of the working fluid or working medium as possible and therefore high efficiency, special materials and cooling techniques are employed for the vanes of the turbine. For example, in modern gas turbines, cooling ducts through which cooling medium flows are drawn through the guide vanes and moving blades of the turbine which are thus cooled. For this purpose, cooling medium is routed in a predefined way through cooling ducts and cooling cavities in the turbine vane (that is to say, through the guide vane and/or moving blade) and/or is conducted outward at various locations on the vane through corresponding orifices (bores, slots). A cooled vane of this type is described in U.S. Pat. No. 5,813,835, which is herein incorporated by reference. The turbine vanes, together with the cover plates, are consequently exposed to the high temperatures and temperature gradients under operating conditions, thus leading to their use induced wear.
A treatment of seals, necessary on account of the abovementioned cooling technology, of a turbine vane worn in this respect as a result of use is known, for example, from CH 701 031 A1. This publication teaches how seals damaged in this way can be exchanged.
However, during operation, damage to other parts of the turbine vane can also occur. On account of the thermal and mechanical load, the head of the guide vane and/or moving blade of a turbine may be deformed. In particular, those regions of the cover plates which span a cavity may bulge outward, that is to say opposite to the direction from the respective cover plate to the vane root. Such bulging and therefore the loss of the original shape may lead to increased or undesirable rubbing contact between the cover plate or a covering layer covering the cover plate and the opposite surface moving in relation to this and to corresponding wear making refurbishment necessary.
In an inspection of the vane, the vane has to be reworked in such a way that, when reinstalled, it satisfies the stringent requirements as to stability conditions and to fitting accuracy.